This invention relates generally to computed tomograph (CT) imaging and, more particularly, to reducing x-ray exposure and improving x-ray efficiency in a multislice CT system.
In at least some computed tomograph (CT) imaging system configurations, an x-ray source projects a fan-shaped beam which is collimated to lie within an X-Y plane of a Cartesian coordinate system and generally referred to as the "imaging plane". The x-ray beam passes through the object being imaged, such as a patient. The beam, after being attenuated by the object, impinges upon an array of radiation detectors. The intensity of the attenuated beam radiation received at the detector array is dependent upon the attenuation of the x-ray beam by the object. Each detector element of the array produces a separate electrical signal that is a measurement of the beam attenuation at the detector location. The attenuation measurements from all the detectors are acquired separately to produce a transmission profile.
In known third generation CT systems, the x-ray source and the detector array are rotated with a gantry within the imaging plane and around the object to be imaged so that the angle at which the x-ray beam intersects the object constantly changes. X-ray sources typically include x-ray tubes, which emit the x-ray beam at a focal spot. X-ray detectors typically include a post patient collimator for collimating scattered x-ray beams received at the detector. A scintillator is located adjacent the post patient collimator, and photodiodes are positioned adjacent the scintillator.
Multislice CT systems are used to obtain data for an increased number of slices during a scan. Known multislice systems typically include detectors generally known as 3-D detectors. With such 3-D detectors, a plurality of detector elements form separate channels arranged in columns and rows. Each row of detectors forms a separate slice. For example, a two slice detector has two rows of detector elements, and a four slice detector has four rows of detector elements. During a multislice scan, multiple rows of detector cells are simultaneously impinged by the x-ray beam, and therefore data for several slices is obtained.
In known CT systems, the x-ray beam from the x-ray source is projected through a pre-patient collimating device, or collimator, that defines the x-ray beam profile in the patient axis, or z-axis. The collimator includes x-ray absorbing material with an aperture therein for restricting the x-ray beam. The process of restricting the x-ray beam to the desired fan beam profile is termed "collimation".
With respect to restricting the x-ray beam, known collimators typically include two opposing metallic blades that may be opened and closed to change the aperture width. The fan beam "thickness", as measured along the z-axis, can be selected by adjusting the blade orientation. The blades also may be moved in a same direction to displace the centerline of the aperture. Changing the aperture centerline changes the fan beam angle with respect to the z-axis.
In multislice CT systems, it is desirable to have only the umbra of the x-ray beam fall on the detector cells. Although the x-ray beam can initially be collimated so that the penumbra does not fall on the detector cells, thermal expansion of the x-ray source causes z-axis movement of the x-ray source focal spot, causing the x-ray beam not to be centered on the detector. Slice thickness may also be affected by misalignment of the focal spot in the y-dimension, target angle, and the size of the focal spot. Additionally, mechanical forces due to centripetal loading increases as the gantry is rotated, which result in focal spot and fan beam movement. As the fan beam moves, it is possible that at least part of the penumbra will fall on the detector cells. Movement of the fan beam changes the strength of signals from the detector array cells. Such fan beam movement may cause differential gain errors and result in severe ring, band and center artifacts.
Accordingly, it would be desirable to provide a CT system that enables selection of the number and thickness of slices and improve x-ray beam stability and efficiency to reduce patient dosage.